Tapered current limit protection for d. c. motor

ABSTRACT

A tapered current limit control for a thyristor-powered direct current shunt motor lowers the current limit level in speed ranges wherein the current carrying ability of the motor is reduced. A variable transconductance analog multiplier is inserted between a speed/voltage regulator which derives the current reference and a current regulator which receives the modified current reference and controls the firing circuit for the thyristors, and a modifying circuit including an absolute value amplifier responsive to motor speed attenuates the gain of the analog multiplier when motor speed exceeds predetermined magnitudes in both the forward and reverse directions of motor rotation.

United States Patent H 1 Larson [54] TAPERED CURRENT LIMIT PROTECTIONFOR C. MOTOR 1 Feb. 13,1973

Primary Examiner-Bernard A. Gilheany Assistant ExaminerThomas Langer[75] inventor: Dale E. Larson, Milwaukee, Wis. A'wmey Lee Kaiser,

[73] Assignee: Allis-Chalmers Corporation, Mil- [57] ABSTRACT waukee Atapered current limit control for a thyristor- Filedl 1972 powereddirect current shunt motor lowers the current limit level in speedranges wherein the current carry- 1, [21] P No 229389 ing ability of themotor is reduced. A variable transconductance analog multiplier isinserted between a U.S. Cl- .Q peed/voltage regulator which derives thecurrent Cl. .1102) reference and a current regulator receives the Fleldof Search modified current reference and controls the firing circuit forthe thyristors, and a modifying circuit includ- References cltled ing anabsolute value amplifier responsive to motor UNITED STATES PATENTS speedattenuates the gain of the analog multiplier when motor speed exceedspredetermined magnitudes 3,295,040 12/1966' Schiemann ..3l8/332 in boththe forward and reverse directions of motor Nye rotatior 3,588,6556/1971 Egan ..3l8/345 11 Claims, 3 Drawing Figures i0- mir+ 5? Mr 60 2a4? 14 i 526:0/ 1 /44 I ammvr F/F/A/ 70474636 NWT/P1 l6? #5601470)?C/Fdfl/ 7" 175645470? 7 v C e5 l Z" A 10 Am v 6 Z. Ew -#2 F C Ffi/t 74,5!

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l v e (5P0 FHA r TAPERED CURRENT LIMIT PROTECTION FOR D. C. MOTOR Thisinvention relates to current limit protection for direct current motors.

BACKGROUND OF THE INVENTION D. C motor drives usually require a currentlimit control to protect the D. C. motor from overcurrent. Known currentlimit circuits regulate at the same value of current over the entireoperating voltage/speed range of the motor. However, some applicationsrequire additional limiting control to protect a motor, for example,when the motor is operated above rated voltage or into the fieldweakening range. As motor speed rises, the speed of the commutator barspast the brushes increases. A tendency to flash the motor occurs underhigh speed conditions, and consequently the currentcarrying ability ofthe motor is limited by commutation problems at high speeds.

Motor control systems for thyristor-powdered D. C. motors usuallyinclude a speed/voltage regulator that feeds into a current regulatorwhich is responsive to armature current flowing through the motor andcontrols the firing-circuit for the thyristors that supply armaturecurrent to the motor. The speed/voltage regulator accepts a referenceinput signal and compares it with a speed feedback signal proportionalto either motor speed or voltage, and the difference is amplified toproduce a current reference to the current regulator stage. This currentreference is compared with a current feedback signal proportional todrive armature current, and the error is amplified'in the currentregulator to produce a voltage which is fed to the phase con trolcircuitry that determines the firing angle of the thyristors. As thephase angle is increased, the D. C. output voltage of the thyristorbridge increases and consequently the SCRs are phased forward in orderto increase the D. C. armature current to the motor.

The gain of the speed/voltage regulator in such a conventional motorcontrol system is linear over the normal speed/voltage range, andcurrent limit is established when the speed/voltage regulator saturatesso that the current reference signal (which it feeds to the currentregulator) can no'longer rise and, consequently, the motor armaturecurrent is limited. Such current limit circuit regulates at the samevalue of current over the entire operating voltage/speed range of themotor and cannot-adequately protect a motor operated above ratedvoltageor into the field weakening range.

Prior art current limit controls are known using current feedback ina'closed loop which employed both a nonlinear impedance responsive tomotor speed plus zener diode spillover to taper the current limit as afunction of motor speed. However, as the magnitude of current feedbackvaried, the response of the current regulator also changed. Such acurrent control loop must have extremely high gain to limit effectively,and this high gain resulted in stability problems. Further, although thespillover was instantaneous, the nonlinear element had a long time lagafter a voltage proportional to motor speed was applied thereto with theresult that the current limit tapering varied from the desired responsewhen the motor was rapidly accelerated or decelerated. Further, thenonlinear impedance was not adjustable, and calibration of the currentlimit magnitude was difficult.

OBJECTS OF THE INVENTION It is an'object of the invention to provide aD. C. motor control system having improved current limit means whichprotects the motor in speed ranges wherein the current-carrying abilityof the motor is reduced.

Anotherobject of the invention is to provide improved tapered currentlimit protection for a D. C. motor which automatically reduces thecurrent limit level to a safe value for the motor as its overcurrencapability is reduced.

Still another object of the invention is to provide improved taperedcurrent limit protection for a D. C. motor which has effectively no timelag so that response is instantaneous and tapering is always in effeet.

A further object of the invention is to provide improved tapered currentlimit protection for a D. C. motor which continuously regulatesdynamically so that the stability problems encountered in prior artdevices are avoided.

SUMMARY OF THE INVENTION In accordance with the invention, the currentlimit for a thyristor-powered direct current shunt motor is lowered inspeed ranges of reduced current-carrying ability by a variabletransconductance analog multiplier connected between a speed/voltageregulator which derives the current reference and a current regulatorwhich receives the modified current reference and controls the firingangles of the thyristors. A modifying circuit including an absolutevalue amplifier responsive to motor speed attenuates the gain of theanalog multiplier when motor speed exceeds a predetermined magnitude.

DESCRIPTION OF THE DRAWINGS These and other objects and advantages ofthe invention will be more readily understood from the followingdetailed description when considered in conjunction with theaccompanying drawing wherein:

FIG. 1 is a speed-current graph showing the current carrying capabilityof a typical D. C. shunt motor to be protected by the invention;

FIG. 2 is a block diagram of a preferred embodiment of a motor controlsystem in accordance with the invention; and

FIG. 3 is a schematic circuit diagram of the tapered current limitportion of the motor control system of FIG. 2.

DETAILED DESCRIPTION percent current only up to approximately 35 percentof base speed but that itscurrent capability reduces to In accordancewith the 200 percent of rated armature current at double voltage(speed).

FIG. 2 is a block diagram of a tapered current limit motor controlsystem embodying the invention which protects a D. C. shunt motor havingcharacteristics similar to those of FIG. 1 in such speed ranges whereinits current carrying ability .is reduced. Motor armature is connected inseries with a suitable thyristor power supply, shown symbolically as asingle SCR l2, and an alternating current source 14. One terminal of thealternating current source 14 is connected through a current'sensor 16(shown in the drawing as a resistance) to one terminal of the motorarmature 10, and the other terminal of motor armature 10 is connected tothe cathode of thyristor 12.

The speed reference signal for the motor control system may be derivedfrom -a manual rheostat or a digital computer or a process controller(none of which are shown) and is applied to input terminal REF of aspeed/voltage regulator SR. THe other input to speed/voltage regulatorSR is a speed feedback voltage 7 signal e, on a lead SPD FBK from atachometer TACl-l operatively connected to motor armature l0.Speed/voltage regulator SR has a comparator stage (not shown) whichpreferably includes a summing operational amplifier that compares thespeed feedback signal e, on conductor SPU FBK with the reference signalin input REF and amplifies the difference speed error signal when thetwo signals on leads SPD FBK and REF do not cancel. The output of thespeed/voltage regulator SR varies linearly in both the positive andnegative directions until it reaches a point of saturation as shown bythe curve adjacent the output from the block and is the currentreference signal e which is applied directly to the current regulator ofknown motor control systems. When speed regulator SR saturates (or islimited), current limit is developed since the output voltage is themaximum current reference signal that can be applied to currentregulator CR.

invention the current reference signal e from speed/voltage regulator SRis one input to a variabletransconductance analog multiplier MULT whichpreferably has the transfer equation xy,/l0 where the x input is thecurrent reference signal e, from speed, regulator SR and the y input isthe output voltage signal e, from a modifying circuit 18 which receivesthe speed feedback signal e, from a tachometer TACl-l as an input.Analog multiplier MULT may be a differential input multiplier/dividersuch as the type sold by Burr-Brown Research Corporation having thetransfer equation:

connected in the multiply mode and provides a DC. output voltage eproportional to the algebraic products of thex (current reference e andy(e from modifying circuit 18) inputs. i v

When speed/voltage regulator SR reaches its output limit due tosaturation or zener spillover type limiting, the current referencesignal e is a maximum and automaticallyestablishes current limit. Inorder to generate a tapered current limit" which approximates thecurrent capability curve of FIG. 1, the curre nt reference" signal e(and hence armature current) is dynamically modified by analogmultiplier MULT as motor speed increases.

Modifying circuit 18 receives the speed feedback signal e, on lead SPDFBK as an input and derives an output signal e which varies with speedin accordance with the U-shaped curve shown adjacent the output from theblock so that the output signal e, is constant over a predeterminedportion of the speed range and is attenuated, preferably linearly, atgiven motor speeds above percent rated speed as described hereinafter.

Multiplier MULT preforms the multiplication function (xy/'l0) and, ineffect, superimposes the current reference signal e from speed/voltageregulator SR and the U-shaped output characteristics e, from modifyingcircuit 18 having two break points F and R therein. inasmuch as theoutput of modifying circuit 18 is constant between the two breakpoints,multiplier MULT operates at unity gain with respect to the speed errorsignal e over the speed/voltage range between the two break points F andR.

FIG. 1 shows that the motor may be operated at 300 percent of armaturecurrent at 100 percent rated (or base) speed and that at double ratedspeed the armature current should be reduced (tapered) to percent. Itwill thus be seen that the forward and reverse break points F and R onthe output versus speed characteristic from modifying circuit 18 shouldoccur at 100 percent speed so that modifying circuit 18 reduces, orattenuates, its output beyond 100 percent speed. The gain of multiplierMULT is thus unity between the forward and reverse speed break points Fand R and droops to a value less than unity beyond them. The multiplierMULT superimposes the output e, from modifying circuit 18 upon thecurrent reference signal e from speed/voltage regulator SR and generatesa modified current reference e, which varies with speed in accordancewith the characteristic shown adjacent the output from the modifyingcircuit block.

The modified current reference e, from analog multiplier MULT is oneinput to a current regulator CR which receives on its other input acurrent feedback" voltage signal on a lead CUR FBK developed acrosscurrent sensor 16 by the motor armature current flowing therethrough.Current regulator CR has a comparator stage which preferably is asumming operational amplifier (not shown), and when the modified currentreference 2 and current feedback signals do not cancel, the differenceis amplified by current regulator CR and its output controls a firingcircuit 20 to change the firing phase angle of SCR 12, thereby causingmotor armature current to change until the difference between signals e,and CUR FBK is zero. Thus the motor armature current is made to matchthe current indicated by speed/voltage regulator SR as dynamicallymodified by multiplier MULT.

FIG. 3 illustrates in greater detail the tapered current limit portionof the motor control system including modifying circuit 18 andmultiplier MULT. The speed regulator output signal e may be adjusted bya rheostat [RH to the proper level for multiplier MULT, typically 10volts. The output signal e, from multiplier MULT is attenuated by arheostat 2Rl-l to a modified current reference level e, input to currentregulator CR which established armature current at the desired currentlimit magnitude, i.e., 300 percent of rated current, when speedregulator SR is saturated and the output e from modifying circuit 18 isa maximum. in other words, rheostat 2RH sets the initial current limitbefore tapering begins.

If the D. C. drive is reversing in direction, the current referencesignal e from speed regulator SR and the modified current referencesignal 2,, from multiplier MULT must also reverse. However, the outputsignal e, from modifying circuit 18 must not reverse in order tomaintain the proper polarity of the current reference signals e and e,,and modifying circuit 18 includes two operational amplifiers Al and A2which together comprise an absolute value amplifier whose output isalways of the same polarity regardless of the polarity of the inputsignal thereto.

Amplifier A2 is a summing amplifier and receives a signal on itsinverting input through a resistance R] from the slider of a rheostatSRH having its ends connected across the and terminals of a suitable D.C. power supply. The slider of rheostat SRH is initially adjusted toapply a positive signal through resistance R1 to the inverting input ofamplifier A2 so that amplifier A2 is in negative saturation and providesa constant output shown by the horizontal portion of the U-shaped speedoutput characteristic e shown adjacent the modifying circuit block inFIG. 2 and adjacent rheostat 6Rl-l in FIG. 3. The output of amplifier A2is attenuated by a rheostat 6RH whose slider derives the output e frommodifying circuit 18, and rheostat 6RH is adjusted so that e is theproper value (typically -10 volts) for input to multiplier MULT whenamplifier A2 is saturated. Under such conditions the gain of analogmultiplier MULT will be unity and the current reference e, output fromthe multiplier MULT will be directly proportional to the input e derivedfrom rheostat lRl-l.

The speed feedback signal e, from tachometer TACH is coupled through arheostat 3RH and a resistance R2 in series to the inverting input ofsumming amplifier A2. When motor armature 10 is rotating forward, thespeed feedback signal e is negative, and is coupled through rheostat 3RHand resistance R2 to the inverting input of amplifier A2 and tends tocancel the positive bias signal from rheostat SRH. Rheostats SRl-l andSRH determine when taper begins, i.e., the forward break point F, andrheostats SRH and 3RH are adjusted so that summing amplifier A2 comesout of saturation at 100 percent rated speed. Above 100 percent rated vspeed the negative speed feedback signal e, applied to the invertinginput of amplifier A2 will be amplified and cause the output ofamplifier A2 to become more positive to thus generate the taperedportion FG of the speed-output characteristics e which slopes upwardlyat a positive'angle from forward break point F. The speed feedbacksignale, is attenuated by rheostat 3RH which defines the slope of thetapered portion F6.

The output of amplifier Al is coupled to the inverting input of summingamplifier A2 through a resistor R3 whose resistance is-preferablyone-half the value of resistor R1 or R2. The speed feedback signal e, iscoupled through a rheostat 4RH and a resistance R4 to the invertinginput of amplifier Al. The feedback impedance circuit z of amplifier Alincludes diode means (not shown) which blocks one polarity of signal andprevents amplifier Al from responding to negative values of speedfeedback signal e,. In other words, amplifier A1 is directional and onlyamplifies the positive signals e, generated by tachometer TACH when themotor is rotating in the reverse direction. The positive polaritysignals e, are inverted by amplifier A1, and the output from amplifierAl increases in a negative direction with speed when the motor isrotating in the reverse direction as shown in the straight linecharacteristic adjacent the output of amplifier Al. Amplifier A2 remainsin saturation during reverse rotation of the motor from zero speed untilpercent rated speed is attained, at which point the negative output fromamplifier A1 cancels the positive bias from rheostat SRH and the speedsignal from 3RH sufficiently so that amplifier A2 comes out ofsaturation, thereby defining the reverse break point R of the e -speedcharacteristic. At reverse motor speeds above 100 percent rated speed,the negative output signals from amplifier A1 are amplified by summingamplifier A2 to form the tapered portion R] of the e -speed curve whichslopes upwardly at a negative angle. Rheostat 4RH sets the gain throughamplifier Al for the reverse speed signal and together with rheostats3RH and SRH determinethe break point R for reverse motor rotation.

If closer approximation of the-nonlinear shape the curve of FIG. 1 isrequired, suitable nonlinear elements (not shown) may be inserted in theZ feedback impedance circuit of amplifier A2 to provide the desiredshaping of thee speed characteristic.

Analog amplifier MULT has no appreciable time lag, and consequently,tapering of current limit is always in effect even during rapidacceleration and deceleration of the motor. The forward and reversebreak points are easily adjusted by rheostats 3RH, 4RH and SRH. Further,inasmuch as the tapering is accomplished in the forward minor currentloop by modification of the current reference e the tapering cannot beoverridden. Further, inasmuch as the current reference e is modified bythe speed feedback signal e, to accomplish tapered current limit, thecurrent regulator CR need not have high gain as was required by knowncurrent limit devices using nonlinear elements and zener spilloverdevices to accomplish tapered current limit, thus avoiding theinstability and time delay problems encountered in prior art devices.

It should be understood that I do not intend to be limited to theparticular embodiment shown and described for many modifications andvariations thereof will be readily apparent to those skilled in the art.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a control systemfor a direct current motor operative with athyristor power supply, the combination of:

a voltage reference regulator having a maximum output, a currentregulator, thyristor firing circuit means operative with said thyristorpower supply and coupled to the output of said current regulator,

an analog multiplier between the output of said voltage referenceregulator and the input of said current regulator, and

means coupled to said analog multiplier and responsive to the speed ofsaid motor for attenuating the gain of said analog multiplier when thespeed of said motor exceeds a predetermined magnitude. 2. In a controlsystem in accordance with claim 1 wherein the current carrying abilityof said direct current motor tapers off with increase in motor speedalong a speed-current characteristic, and said gain attenuating meansdecreases the gain of said analog multiplier with increase in motorspeed above said predetermined magnitude along a curve whichapproximates said speed-current characteristic.

3. In a control system in accordance with claim 1 including means forgenerating a speed feedback signal which is a function of the speed ofsaid motor, and wherein said gain attenuating means includes an absolutevalue amplifier receiving said speed feedback signal as an. input andwhose gain is afunction of the magnitude but independent of the.polarity of said speed feedback signal.

4'. In a control system in accordance with claim 3 wherein said absolutevalue amplifier includes a summing operational amplifier having saidspeed feedback signal coupled to the'inverting input thereof, and meansfor applying adirect current bias signal to said inverting input to biassaid summing amplifier into saturation, whereby the output of saidsumming amplifier is constant until said speed feedback signal cancelssaid bias signal sufficiently to bring said summing amplifier out ofsaturation.

5. In a control system in accordance with claim 4 wherein said absolutevalue amplifier includes a second operational amplifier having saidfeedback signal coupled to the inverting input thereof and having itsoutput coupled to the inverting input of said summing amplifier, andmeans for preventing said second operational amplifier from respondingto one polarity of said speed feedback signal.

6. In a control system for a direct current motor energized from athyristor power supply regulated by gating signals from a firing circuitwhose phase angles are a function of an input to said firing circuit,

feedback means for generating a speed feedback signal which is afunction of the speed of said motor,

current feedback means for generating a current feedback signal which isa function of the armature current in said motor,

a reference signal source,

a speed regulator for comparing said reference and speed feedbacksignals and having a maximum a current regulator for comparing theoutput from said analog multipler with said current feedback signal andfor supplying a current error signal to said firing circuit which is afunction of their difference, and

modifying circuit means receiving said speed feedback signal as an inputfor attenuating the gain of said analog amplifier when the speed of saidmotor exceeds a predetermined magnitude in the forward direction andalso when it exceeds a predetermined magnitude in the reverse direction.7. In a con rol system m accordance with claim 6 amplifier intosaturation when said motor is at standstill, whereby the output of saidsumming amplifier is constant until said speed feedback signal .cancelssaid bias signal sufficiently to bring said summing amplifier out ofsaturation.

9. In a control system in accordance with claim 8 wherein said absolutevalue amplifier also includes a second operational amplifier having saidspeed feedback signal coupled to the inverting input thereof and havingits output coupled to said inverting input of said summing amplifier,and means for preventing said second operational amplifier fromresponding to one polarity of said speed feedback signal.

10. In a control system in accordance with claim 9 wherein said directcurrent bias signal is of opposite polarity to said speed feedbacksignal when said motor is rotating in the forward direction and said onepolarity of said speed feedback signal to which said second operationalamplifier does not respond is generated when said motor is rotating inthe forward direction.

11. In a control system in accordance with claim 6 wherein saidmodifying circuit means generates a substantially constant output signalwhen said speed feedback signal is below a predetermined first valuewhen said motor is rotating forward and also when said speed feedbacksignal is below a second predetermined value when said motor isreversed, and wherein said modifying circuit generates an output signallower than said constant output signal when said speed feedback signalis greater than said first predetermined value when said motor isrotating forward or is greater than said second predetermined value whensaid motor is reversed.

' t I I III

1. In a control system for a direct current motor operative with athyristor power supply, the combination of: a voltage referenceregulator having a maximum output, a current regulator, thyristor firingcircuit means operative with said thyristor power supply and coupled tothe output of said current regulator, an analog multiplier between theoutput of said voltage reference regulator and the input of said currentregulator, and means coupled to said analog multiplier and responsive tothe speed of said motor for attenuating the gain of said analogmultiplier when the speed of said motor exceeds a predeterminedmagnitude.
 1. In a control system for a direct current motor operativewith a thyristor power supply, the combination of: a voltage referenceregulator having a maximum output, a current regulator, thyristor firingcircuit means operative with said thyristor power supply and coupled tothe output of said current regulator, an analog multiplier between theoutput of said voltage reference regulator and the input of said currentregulator, and means coupled to said analog multiplier and responsive tothe speed of said motor for attenuating the gain of said analogmultiplier when the speed of said motor exceeds a predeterminedmagnitude.
 2. In a control system in accordance with claim 1 wherein thecurrent carrying ability of said direct current motor tapers off withincrease in motor speed along a speed-current characteristic, and saidgain attenuating means decreases the gain of said analog multiplier withincrease in motor speed above said predetermined magnitude along a curvewhich approximates said speed-current characteristic.
 3. In a controlsystem in accordance with claim 1 including means for generating a speedfeedback signal which is a function of the speed of said motor, andwherein said gain attenuating means includes an absolute value amplifierreceiving said speed feedback signal as an input and whose gain is afunction of the magnitude but independent of the polarity of said speedfeedback signal.
 4. In a control system in accordance with claim 3wherein said absolute value amplifier includes a summing operationalamplifier having said speed feedback signal coupled to the invertinginput thereof, and means for applying a direct current bias signal tosaid inverting input to bias said summing amplifier into saturation,whereby the output of said summing amplifier is constant until saidspeed feedback signal cancels said bias signal sufficiently to bringsaid summing amplifier out of saturation.
 5. In a control system inaccordance with claim 4 wherein said absolute value amplifier includes asecond operational amplifier having said feedback signal coupled to theinverting input thereof and having its output coupled to the invertinginput of said summing amplifier, and means for preventing said secondoperational amplifier from responding to one polarity of said speedfeedback signal.
 6. In a control system for a direct current motorenergized from a thyristor power supply regulated by gating signals froma firing circuit whose phase angles are a function of an input to saidfiring circuit, feedback means for generating a speed feedback signalwhich is a function of the speed of said motor, current feedback meansfor generating a current feedback signal which is a function of thearmature current in said motor, a reference signal source, a speedregulator for comparing said reference and speed feedback signals andhaving a maximum output, an analog multiplier receiving the output ofsaid speed regulator on one input thereto, a current regulator forcomparing the output from said analog multipler with said currentfeedback signal and for supplying a current error signal to said firingcircuit which is a Function of their difference, and modifying circuitmeans receiving said speed feedback signal as an input for attenuatingthe gain of said analog amplifier when the speed of said motor exceeds apredetermined magnitude in the forward direction and also when itexceeds a predetermined magnitude in the reverse direction.
 7. In acontrol system in accordance with claim 6 wherein said modifying circuitmeans includes an absolute value amplifier whose output is a function ofthe magnitude but independent of the polarity of the speed feedbacksignal input thereto.
 8. In a control system in accordance with claim 7wherein said absolute value amplifier includes a summing operationalamplifier, means to couple said speed feedback signal to the invertinginput of said summing amplifier, and means to apply a direct currentbias signal to said inverting input to bias said summing amplifier intosaturation when said motor is at standstill, whereby the output of saidsumming amplifier is constant until said speed feedback signal cancelssaid bias signal sufficiently to bring said summing amplifier out ofsaturation.
 9. In a control system in accordance with claim 8 whereinsaid absolute value amplifier also includes a second operationalamplifier having said speed feedback signal coupled to the invertinginput thereof and having its output coupled to said inverting input ofsaid summing amplifier, and means for preventing said second operationalamplifier from responding to one polarity of said speed feedback signal.10. In a control system in accordance with claim 9 wherein said directcurrent bias signal is of opposite polarity to said speed feedbacksignal when said motor is rotating in the forward direction and said onepolarity of said speed feedback signal to which said second operationalamplifier does not respond is generated when said motor is rotating inthe forward direction.